This research is aimed at understanding the mechanism of charge transfer across biological interfaces. This interfacial region of finite depth is of importance in biological processes which range from energy transduction to cell-cell interactions. The kinetics of selected photoinitiated charge transfer reactions localized at the interface are used as probes of this region of the lipid bilayer. The electric field dependence of the kinetics of charged hydrophobic pigments will be used to study properties polarity and dielectric gradients) of this region. Rigid donor-acceptor complexes specifically orientable at the bilayer-water interface will be synthesized and used to probe the interface on the short (ns) time scale. Proton transfers in this region will be followed by suitable donors or acceptors, e.g. quinones. The cross membrane transit time of both charged and uncharged species will be measured by these photoinitiated single interface charge transfer reactions and suitable redox, pH or ion gradients. The measurements are made with fast voltage and current amplifiers and very fast digitizers. The light sources are flash lamp and N2 laser pumped dye lasers.